This application claims priority from U.S. Provisional application Ser. No. 60/322,606 filed on Sep. 17, 2001.
The present invention involves forming an envelope or coating over a core that may expel gases and vapors as it is heated during the production process. In many cases, the production of these gases by a wood or other substrate causes poor adhesion of the coating. U.S. Pat. No. 3,432,885 xe2x80x9cZaninixe2x80x9d, which is hereby incorporated by reference, involves a continuous process for forming an envelope or coating over a core that may expel gases and vapors as it is heated during the production process. Zanini provides bores 22 for the aspiration of gases and vapors that are driven off during the heating, allowing those gases to vent to atmosphere.
The purpose of the present invention is to improve over Zanini by removing the potential gases and vapors before applying the hot envelope or coating. This removes much of the material that could prevent the coating from attaching uniformly to the surface and creates an insulating layer between the internal portions of the substrate that may still have liquid or resin to prevent those internal materials from vaporizing during the application of the hot surface coating. In most embodiments of the present invention, the substrate is subjected to heating and to a high speed, high volume air stream, which may be created by a vacuum or by a positive pressure air source, followed closely thereafter by the coating. The heat and/or the air stream drive off the gases that would have been formed during the coating process. The air stream also cools down the substrate, so that the internal portions of the substrate will not reach high enough temperatures during the coating process to cause further expulsion of gases.
Wood is used as an example, but this process is applicable to any material which is to become a substrate in a thermal laminating process where that substrate has a tendency to absorb and or contain a moisture or other volatile materials such as resin (sap) which might be volatilized at relatively low temperatures (less than 340 degrees Fahrenheit). We will refer to these materials as hygroscopic materials.
While this process may potentially have the greatest application to a continuous process such as co-extrusion, it is not limited to a co-extrusion process. It could also be used to coat a tabletop or a desktop, a round circle on some surface, or for many other coating arrangements.
Generally, it is not necessary to use supplemental adhesives, as the thermoplastic material itself when heated to the liquid state tends to create an adhesive bond with compatible surfaces. While many hygroscopic materials exist, wood just happens to be a worst case example, because it not only has water, but it also has sap or other chemicals which vaporize during the coating process.